Generally more flow is better, Ehiems are not that good for flow unless it's the new super one or the older 2250/2260's.

If you want more CO2 mist in the tank, place the venturi close as you can to the return outlet. Less distance and travel time in the gas phase.

If the tank is very large and you want a lot of dissolved CO2 quickly, then placing the venturi inside the sump with it's own dedicated powerhead(however large you need) and feeding the outflow to the return pump's suction side.

That (rather than pressure drop differences due to placement along the line) is more the issue.

Regards,
Tom Barr